An important factor that The present invention relates to static random access memories (“SRAMs”), and more particularly to SRAMs having circuits for altering a voltage level supplied thereto. In addition, aspects of the invention relate to methods of operating an SRAM in which a voltage level supplied thereto is altered.
SRAMs are uniquely suited to the functions they serve within processors and other devices for storage of data to which fast (low cycle time) and ready (low latency) access is desired. Certain types of storage within processors are almost always implemented using SRAMs, such as cache memories, control stores, buffer memories, instruction pipelines and data pipelines including input output interfaces and buffers for direct memory access (“DMA”) interfaces. In addition, certain storage used for communication interfaces, e.g., network adapter buffers and so on, also utilize SRAMs for speed and low latency. Since SRAMs are frequently incorporated into chips on which other functions are implemented, e.g., processors (also referred to variously as microprocessors and central processing units (“CPUs”)), they must perform at operating conditions as difficult as those that the processors must tolerate. In particular, SRAMs must perform within the same broad range of operating temperatures as processors and must be capable of tolerating fluctuations in supply voltages, e.g., noise disturbance, to the same extent as processors. Moreover, the sizes of SRAMs and SRAMs incorporated into chips having other functions are increasing. It is not uncommon for SRAMs to store many tens of millions of bits, even for SRAMs which are incorporated into other chips such as processors. In addition, to satisfy a growing demand for application specific integrated circuits (“ASICs”), it is desirable to provide SRAM macros (“macros” being functional modules) with large capacities capable of incorporation into multi-function chips, despite the scaling of the transistors and voltages used therein to unprecedented small sizes and values.
Storage cells in an SRAM or “SRAM cells” are arranged in an array of SRAM cells or “SRAM array.” Wordlines (“WL”) run in a direction of rows across an SRAM array. Bitlines run in a direction of columns across the SRAM array. Typically, one wordline WL is connected to each cell of a row of cells in the SRAM array. Typically, two bitlines carrying complementary signals are connected to each cell in a column of cells in the SRAM array, with one of the bitlines carrying a “true” signal representing the actual state of a bit signal and the other bitline carrying a “complementary” signal representing an inverted version of the bit signal.
As the size of each cell within the SRAM array decreases with later generations of SRAMs, the threshold voltage of field effect transistors (“FETs”) used in each cell is subject to increased variability. In addition, the voltage level at which power is supplied to the SRAM array is reduced or “scaled” with the introduction of new generations of SRAMs.
The increased variability of the threshold voltage of the FETs of cells of an SRAM and the reduced power supply voltage level make it harder to guarantee that certain margins of error are maintained during operation of the SRAM. Such margins of error have a direct bearing on the SRAM's ability to maintain the integrity of the data stored therein. Clearly, there is a requirement that an extremely miniscule amount of such errors occur during operation of an SRAM. The maximum tolerable error or upper limit for such error is often measured in terms of a number of “sigma”, sigma representing the standard deviation in a distribution curve representing the occurrence of such errors. As currently manufactured, SRAMs can have many millions of cells per SRAM array, the upper limit typically is set at one or two errors per the SRAM array. Stated another way, the maximum tolerable rate of error is set at a level such as one or two parts per billion, for example. This translates approximately to 5.2 sigma.
Margins of error which need to be maintained in the SRAM include access disturb margin (“ADM”) and write margin (“WRM”). The state of a bit stored in a cell of an SRAM is more likely to undergo a spontaneous inversion when the SRAM cell is partially selected. Access disturb margin (ADM) is a measure of the likelihood that the state of a bit stored in a partially selected cell of the SRAM array will spontaneously change from one state to another, e.g., flip from a “high” to a “low” state when a cell of the SRAM is accessed during a read or write operation. An unselected SRAM cell is “half” selected when a wordline connected to that cell has been activated. In other words, a cell is half selected when another cell connected to the same wordline is accessed for either a read or a write operation. Such “half” selected cells are more susceptible to disturbance during access to the cells within the SRAM because such cells are frequently subject to potential disturbance during such accesses at times when they are not being accessed.
Another margin of error that must be satisfied is the ability to write the state of bit to an SRAM cell, given the strength of the bitline signals supplied to the SRAM cell and the time allotted to do so. Here, it is important that the SRAM cell have sufficient drive current to change from one stable state to another state under the influence of the bitline signals supplied thereto. If the SRAM cell fails to be written with the bit that is provided thereto, data integrity is impacted. WRM pertains to the occurrence of this type of error. Here again, it is important to reduce this type of error to an extremely miniscule amount. WRM, like ADM is typically measured in terms of standard deviations or “sigma” from a center of a distribution of the occurrence of error. As in the above case, WRM should preferably be maintained at a high sigma number, preferably at a sigma value of about 5.2 or more.
The scaling of one or more voltages supplied to each SRAM cell of the SRAM array for each succeeding generation of SRAMs only increases the difficulty of achieving a desired ADM and desired WRM. Various approaches have been suggested for providing high ADM, despite the scaling of the voltages. Increasing the threshold voltage of each n-type FET (“NFET”) and p-type FET or (“PFET”) helps improve ADM. However, such approach potentially worsens WRM. In an SRAM, the available amount of drive current within each SRAM cell is already limited because of the scaling of the voltage. As a result, raising the threshold voltages of the NFET and PFET make the NFET weaker and the PFET stronger. While the SRAM cell becomes more stable under such conditions, writing each SRAM cell becomes more difficult than before.
Moreover, whether or not the threshold voltage of the NFETs is raised or that of the PFETs is lowered, ADM and WRM tend to respond differently to temperature. The sigma value of ADM tends to be higher at lower temperatures, e.g., when a chip containing an SRAM is first turned on or when the chip is running at lower frequency. On the other hand, the sigma value of ADM becomes lower with increasing temperature. On the other hand, the sigma value of WRM tends to be lower at lower temperatures, and the sigma value of the WRM becomes higher with increasing temperature. As transistor sizes are scaled further downward in future generations of SRAMs, it becomes more difficult to achieve desirable sigma values on both ADM and WRM over the range of temperatures in which SRAMs (and processors which incorporate them) are required to operate.
Providing write schemes for SRAM cells having PFET passgates is one goal featured in commonly assigned U.S. Pat. No. 6,549,453 to Wong (“the '453 patent”). In one scheme described in the '453 patent, a voltage provided to a memory cell is adjusted from one level to another during a data writing operation to the memory cell. Circuits are provided by which a supply voltage provided to pull-up devices of the memory cell “floats down” to a lower level during the data writing operation. In addition, in one or more other schemes described therein, pull-down devices of the memory cell are disconnected from ground and allowed to “float up” to a voltage level higher than ground.
From the foregoing discussion, it is clear that new ways are needed to maintain high WRM and ADM in SRAMs despite further scaling of the transistors and voltages used in SRAM cells.